Struggling with Javascript Unit Testing 'Code Challenge: Catching an Error'

Hi Mohammed. I found this quite tough too. I had only done this course a few months back, I must have passed the challenge back then but this time, I struggled so much, I copied the challenge to my computer so that I could experiment freely without suffering that 'Bummer' message. It was good that I did.

The first thing to mention is a key sentence in their Challenge...

Make sure to check for subtraction's specific error message!

So we need to be checking for the specific text ''subtraction only works with numbers!', and we should witness this by specifically ensuring one of the 2 arguments provided to the subtraction function is not a number.

So testing 'offline', I proved to myself that the subtraction worked correctly when provided with 2 numbers.

describe('subtraction', function () {
var subtraction = require('../WHEREVER')   
    it('can correctly subtract 1 from 3', function () {
        expect (subtraction(3, 1)).to.equal(2);
  })
})

The above resulted with...

  subtraction
    √ can correctly subtract 1 from 3

So after a great deal of reading, I replaced one number with a letter and expected the following to work...

describe('subtraction', function () { 
    it('it should state "subtraction only works with numbers!"', function () {
        expect (subtraction('a', 1)).to.throw('subtraction only works with numbers!');
    })
})

But not so... :(

  1) subtraction
       it should state "subtraction only works with numbers!":
     Error: subtraction only works with numbers!
      at Error (native)
      at subtraction (subtraction.js:3:13)
      at Context.<anonymous> (subtraction_spec.js:27:17)

I watched Guil's previous video and sure enough, at around 6:42 he states...

So it looks like the throw method requires me to wrap my function in something that Chai can handle internally. Now, if I didn't do this, then my test spec would always fail, because placeShip will throw an error. Mocha will think that error counts as a test failure, even though the error is really what I expect.

So I just had to wrap the subtraction function inside another function, so as not to confuse Chai like so...

describe('subtraction', function () { 
    it('it will state "subtraction only works with numbers!"'', function () {
        var handler = function () {
            subtraction('a', 1);
        }
        expect (handler).to.throw('subtraction only works with numbers!');
    })
})

So back online, to solve the Treehouse challenge, we just need to replace // YOUR CODE HERE with...

        var handler = function () {
            subtraction('a', 1);
        }
        expect (handler).to.throw('subtraction only works with numbers!');